The NSRI was assisting in the PE 2 EL surf-ski challenge that started in Port Elizabeth on Thursday, spokesman Craig Lambinon said in a statement.
The surf-skiers were all accounted for at the end of day at Woody Cape, 75km east of Port Elizabeth.
Around 8pm at the Woody Cape resort and backpackers lodge, NSRI volunteers were having dinner when they heard cries for help coming from the beach.
Upon investigation they found five people had gone for a swim but were caught in rip-currents and swept out to sea.
“NSRI rescue swimmers went into the surf to go to the aid of the five people, reaching them 100 metres out to sea and managing to rescue three safely and recovering the body of one,” Lambinon said.
The fifth person remained missing in the surf line.
“The three survivors, two males and a female, showed signs of near drowning symptoms and were treated by NSRI medics before being transported to hospital for observation for secondary drowning,” he said.
“SA National Parks rangers also responded to assist. The body of the deceased, a female, has been handed into the care of the police and the forensic pathology services.”
Despite an extensive search no sign of the missing woman was found and the search would resume at first light on Friday morning.
“Police will open an inquest docket for the deceased female and a missing person’s report will be opened for investigation related to the missing female,” Lambinon said.
The NSRI urged the public to be extremely cautious around the coast over the next seven days.
“Spring tide peaks with the full moon on Saturday December 6, bringing higher than normal high tides, lower than normal low tides and hence stronger than normal rip-currents,” said Lambinon.
“As a result of the spring tide the coastline is at its most dangerous during this period because of these stronger than normal rip-currents.”
The five people were British nationals, all part of a volunteer group in the area.